1. Field of the Invention
The present invention is related to a constant current driving circuit, and more particularly, to a constant current driving circuit that utilizes an input/output voltage to control ON/OFF time of a power switch of the constant current driving system to stabilize output current of the constant current driving circuit.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a conventional constant current driving system 100. As illustrated in FIG. 1, the constant current driving system 100 comprises a constant current driving circuit 110, a power switch SW1, an inductor L1, a peak current sensing resistor RSP, and a diode D1. The constant current driving system 100 is utilized to convert an input voltage source VIN to provide output current IOUT and output voltage VOUT to the load X. The constant current driving circuit 110 outputs a switch control signal SSW for controlling the power switch SW1 to be on (closed) or off (open). The constant current driving circuit 110 utilizes sensing voltages VSP+ and VSP− provided by the peak current sensing resistor RSP to control the ON time TON of the power switch SW1; the constant current driving circuit 110 controls the OFF time TOFF of the power switch SW1 to have a fixed duration TP. More specifically, when the difference between the sensing voltages VSP+ and VSP− is lower than a predetermined value, the output current IOUT is determined to be less than a predetermined peak threshold value IP, so the constant current driving circuit 110 outputs the switch control signal SSW representing “ON” (i.e. the switch control signal SSW is asserted) to the power switch SW1 to turn on the power switch SW1, and the input voltage source VIN is then coupled to the inductor L1 for charging the inductor L1, subsequently increasing the output current IOUT; when the difference between the sensing voltages VSP+ and VSP− equals the predetermined value, the output current IOUT is determined to have reached the predetermined peak threshold value IP, so the constant current driving circuit 110 outputs the switch control signal SSW representing “OFF” (i.e. the switch control signal SSW is unasserted) to the power switch SW1 to turn off the power switch SW1 for the inductor L1 to start discharging, subsequently decreasing the output current IOUT; the OFF time TOFF of the power switch SW1 has the fixed duration TP, meaning the constant current driving circuit 110 turns on the power switch SW1 again after the fixed duration TP, so the inductor L1 restarts charging to increase the output current IOUT. This way, by controlling the on/off operation of the power switch SW1 with the constant current driving circuit 110, the charging/discharging operation of the inductor L1 can be controlled for the average value of the output current IOUT to equal a target current ITARGET, achieving the object of constant current control.
Please refer to FIG. 2. FIG. 2 is a timing diagram illustrating the output current IOUT provided by the conventional constant current driving circuit. As illustrated in FIG. 2, when the output current IOUT has not reached the predetermined peak threshold value IP, the switch control signal SSW is represented as “ON” for turning on the power switch SW1 and the output current IOUT is increased (such as during the durations TON1, TON2, TON3, TON4 and TON5); when the output current IOUT has reached the peak threshold value IP, the switch control signal SSW is represented as “OFF”, and the OFF time equals a fixed duration TP (such as the durations TON1, TON2, TON3 and TON4). However, if the load X is varied (e.g. continuously increasing), the discharging rate of the inductor L1 during the OFF time TOFF of the power switch SW1 is increased, and since the OFF time TOFF of the power switch SW1 is of the fixed duration TP, the current is decreased to a further extent. As illustrated in FIG. 2, if the load X increases continuously, the variation of the output current IOUT tends to increase accordingly, say, from ΔIOUT1, ΔIOUT2 and ΔIOUT3 to ΔIOUT4. This way, the ripple current of the output current IOUT is also increased, consequently lowering the stability of the output current IOUT and causing inconvenience to the user.